Dynamics of the Bombay duck (Harpodon nehereus) stock along the northwest coast of India

Abstract

The fishery for the Bombay duck, (Harpodon nehereus) along the northwest coast of India was studied from the data for the annual catch, effort and length- at-age composition for the period 1947 to 1986. Three models, viz., the Beverton and Holt analytical model, Cushing’s stock-recruitment relation, the surplus production models of Schaefer and of Fox and the prediction model of Roff were applied to dEscribe the state of the stock. The length growth parameters; L0 ranged from 37.45 cm (1985) to 60.97 cm (1958), annual k ranged from 0.29 (1958) t0 0.77 (1948) and t, ranged from —0.12 year (1948) to 0.06 year (1985), while the weight growth parameters; W0 ranged from 319.14 g (1948) to 1,910.65 g (1950-51), annual k ranged from 0.26 (1984 and 1985) to 0.85 (1948) and t, ranged from —0.08 year (1948) to 0.03 (1950-51). The length at first capture, l, was 3.00 cm (t0 = 0.13 year) while the length at recruitment, lr was 2.00 cm (tr = 0.08 year). Total annual mortality, Z, ranged from 2.32 (1985) to 8.29 (1948), natural mortality, M, ranged from 0.93 (1950-51) to 1.20 (1948- ‘49) while fishing mortality, F, ranged from 1.32 (1985) to 2.70(1983). Cohort analysis indicated maximum F at 4+age group in 1983 (=2.70) and at 3+ age group in 1984 (=1.77). The stock-recruitment data indicated a linear relation with density dependence (b) ranging from 0.32 to 1.03 with an average of 0.68. The MSY estimated according to the analytical model was found to be 189,844 t which according to the Schaefer and Fox models was 101,158 t and 103,483 t respectively. The prediction model indicated an estimated catch of 92,189 t in the year 2000 if the effort remained a constant.